Writing is a very unique art form in that it gives us the opportunity to do something that no other art form, not even life itself, does: foreshadowing.
Foreshadowing is the fine art of dropping hints, whether subtle or grand, throughout your book to warn your reader of what is yet to come.
All writers do it, whether intentionally or not. Some foreshadows are so small, you may miss them. While others may be blatant and impossible to ignore. Regardless, foreshadowing is actually quite vital to the story - any story, every story.
Speaking for myself (because every writer is different, and I can't speak for anyone else), foreshadowing is something that I unconsciously do. I have literally surprised myself with my own foreshadowing. I write things (numbers, events, locations, etc.) and I don't even realize that it is my mind putting in those details that are vital to the story. For example, Lareina (Unbreakable Hostage) has a license plate that says "x equals two." I wasn't sure why x had to equal two, but it did. As I was writing the story, I just knew that this had to be integrated into the story line. I'm not the greatest mathematician, hence my "job" as a writer. Anyway, it took me a while, but then it all just made sense. All the numbers fell into place. And yes, x does in fact equal two. I wasn't proud of myself (as in "Hey, I'm just THAT good"), but I was excited and impressed that my mind knew about this all along. I didn't expect it to work out. I had always thought that "x equals two" wasn't that important. But it was. It is! I can't tell you any more than that. Unbreakable Hostage is due to come out in February, so just wait till then! ;)
So the point is: as readers, we need to pay attention to every detail; as writers we need to pay attention to every detail. Foreshadowing is fun for both readers and writers. So go, let your mind run free and who knows what kinds of interesting foreshadows you'll end up with!
(Like my crappy Photoshop job? :) )